(1) Field of the Invention
This invention relates to a pneumatic tire for a high speed under a heavy load.
(2) Description of the Prior Art
Here, the wording "for a high speed under a heavy load" shall be understood to mean uses in which at least one of conditions that the load subjected to the tire is remarkably high and that a centrifugal force acting on the ground contact zone of the tire when the tire rotates at a high speed becomes significantly large must be taken into consideration. In a concrete example of such uses, the tire is used under especially severe conditions as in the case of an airplane or similar high speed transport means subjected to a heavy load and it raises a problem whether or not the tire is suitable for such a severe condition.
The tire according to the invention is used under considerably severe conditions which must satisfy at least one of both a wheel weight load in which a value L obtained by dividing the load subjected to the tire by the tire volume is larger than 0.04 and a centrifugal load in which a value H given by v.sup.2 /g.multidot.r were v is a peripheral speed of the tire surface, r is a rotary radius of the tire and g is an acceleration gravity of the tire is larger than 4000.
In FIG. 1 is shown a typical conventional tire used under the above mentioned conditions. As shown in FIG. 1, a tire 1 comprises a side wall portion 2, a tread portion 3, a hump portion 4 having a particularly large thickness, and a case reinforcing ply 5. The entire case reinforcing ply 5 is extended from one of bead cores to the other bead core. If necessary, the case reinforcing ply 5 may be provided at its outermost layer with a breaker for the purpose of improving a cut resistant property of the tire.
In the conventional tire shown in FIG. 1, the case reinforcing ply 5 is composed of plies which are substantially the same in number from the center of the tread portion 3 through the hump portion 4 to the side wall portion 2 and which are uniform in thickness distribution. This thickness distribution is substantially the same even when the case reinforcing ply 5 is provided at its outermost layer with the breaker.
In the case of deriving the thickness distribution by calculation, a cap ply and those plies which are arranged along the bottom of tread grooves are excluded from the calculation.
If the conventional tire constructed as above described is used for high speed under a heavy load as in the present invention, this involves the following problems.
a. Considerably large wheel weight load acting on the tire.
In the conventional tire design, it has been the common practice to increase the amount of air to be fed into the tire, that is, to increase the internal pressure to be fed into the tire or to increase the tire size for the purpose of compensating the considerable increase of the wheel weight acting on the tire. In order to increase the internal pressure to be fed into the tire, if the number of plies of the case reinforcing ply is increased to increase its strength, this strength is determined by the strength at the center of the tread portion 3. As a result, the strength at the hump portion 4 and side wall portion 3 becomes unnecessarily large and hence the weight of the tire is increased and much heat is generated in the tire or the tire becomes expensive. The most simple way of eliminating such problem is to enlarge the tire size. But, this takes up much space and represents a material increase in cost.
b. High speed rotation of the tire.
The conventional tire shown in FIG. 1 is suitable for a simple high speed running. But, if the increased wheel weight load is added to such simple high speed running, a separation failure is induced between the coating rubber of the tread portion 3 on the one hand and the outermost layer or eventually the ply of the case reinforcing ply 5 on the other hand. Most of the separation failure extends circumferentially at substantially the center of the tread portion 3 of the tire. In addition, eccentric wear results in a difference in the ground contact pressure distribution and in a decrease of the ground contact area. In this case, when the tire rotates at a high speed, the mode of the crown portion of the tire becomes different from that of the tire used in general. A considerably large centrifugal force overcomes the tension and rigidity of the case reinforcing ply 5, so that the center of the tread portion 3 is deformed so that the center is projected outwardly in the radial direction of the tire as shown by dotted lines in FIG. 1. That is, the crown radius of curvature is decreased from R to R.sub.1 and this together with difference between the deformation at the ground contact area directly below the load and the deformation at the non-ground contact area released from the wheel weight load function to produce a standing wave, that is, undulating deformation phenomenon due to the fact that within a time that the ground contact deformation of the tire is not yet restored to its original shape the next ground contact deformation is superimposed thereon. In order to prevent such undulating deformation phenomenon, the above mentioned change of the crown radius from R to R.sub.1 should be restrained.
As seen from the above, an improved tire adapted to be used for a high speed under a heavy load without increasing the tire size or without increasing the internal pressure of the tire is now urgently in demand.